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Abstract

A novel scheme of direct electrical contact on vertically aligned silicon nanowire (SiNW) axial p-n junction is demonstrated by means of oblique-angle deposition of slanted indium-tin-oxide (ITO) film for photovoltaic applications. The slanted ITO film exhibits an acceptable resistivity of 1.07x10−3Ω-cm underwent RTA treatment of T = 450°C, and the doping concentration and carrier mobility by Hall measurement amount to 3.7x1020cm−3 and 15.8cm2/V-s, respectively, with an n-type doping polarity. Because of the shadowing effect provided by the SiNWs, the incident ITO vapor-flow is deposited preferentially on the top of SiNWs, which coalesces and eventually forms a nearly continuous film for the subsequent fabrication of grid electrode. Under AM 1.5G normal illumination, our axial p-n junction SiNW solar cell exhibits an open circuit voltage of VOC = 0.56V, and a short circuit current of JSC = 1.54 mA/cm2 with a fill factor of FF = 30%, resulting in a total power conversion efficiency of PEC = 0.26%.

(a) Photographs of three representative samples including bare silicon with (center) and without (left) quarter-wavelength ARC, and the porposed SiNW SC (right). The dimension size of all samples is identical to be 1cm × 1cm. (b) (Top) Measured reflectivity as a function of normal-incident wavelength for all samples. ASTM air mass 1.5 direct and circumsolar solar spectrum (λ = 400–700nm) is also plotted in the figure. (Bottom) Angular-dependent reflectivity (θ = 10°–70°) of all samples measured by an incident light of He-Ne laser (λ = 632.8nm) for both TE and TM polarizations. (c) Optical absorption obtained by A(θ, λ) = 1- R(θ, λ) for all samples.

Electrical performance of the proposed SiNW SC (a) Semi-log plot of current density vs. voltage (J-V) behavior both in the dark and under AM 1.5G simulated sunlight illumination. Insert: photography of actual device with the dimension of 2cmx2cm (right-hand side). J-V curves of SiNW arrays deposited with standard ITO film (left-hand side). The series (RS) and shunt (RSH) resistances, and the reverse current (JR) and ideality factor (n) extracted from the dark J-V curve are also summarized in the figure. (b) The same J-V current plotted in a linear scale. Insert: the corresponding power-density vs. voltage curve.